Limits on Superconductivity in Nanoparticles and Nanowires

Recent experimental results are consistent with the prediction that superconductivity is suppressed in aluminum nanoparticles which are so small (10 nm diameter) that the electronic energy level spacing exceeds the superconducting energy gap in bulk material. Very recent experiments on nanowires (diameter 5–10 nm, length 150 nm) of MoGe indicate that a dissipative phase transition between superconducting and normal behavior occurs when the normal resistance of the wire equals the superconducting quantum resistance h/4e² 6.5 k.     

Vortex imaging in superconducting films by scanning hall probe microscopy

We have used a low noise Scanning Hall Probe Microscope (SHPM) to study vortex structures in superconducting films. The microscope has high magnetic field (2.9×10^–8T/Hz at 77K) and spatial resolution, 0.85m. Magnetic field profiles of single vortices in High T_c YBa₂Cu₃O_7– thin films have been successfully measured and the microscopic penetration depth of the superconductor has been extracted as a function of temperature. Flux penetration into the superconductor has been imaged in real time (8s/frame).     

Hydrazine method of synthesis of γ-Fe2O3 useful in ferrites preparation. Part IV – preparation and characterization of magnesium ferrite, MgFe2O4 from γ-Fe2O3 obtained from hydrazinated iron oxyhydroxides and iron (II) carboxylato-hydrazinates

Electro-magnetic properties and microstructural characterization of MgFe₂O₄ synthesized by a ceramic technique at 1000°C from iron oxides, consisting of mainly -Fe₂O₃ and traces of alpha-Fe₂O₃, prepared from iron ore rejects, are compared with the ferrite obtained from commercial alpha-Fe₂O₃. The sources of -Fe₂O₃ are hydrazinated iron (II) carboxylates and iron oxyhydroxides which autocatalytically decompose giving mainly -Fe₂O₃ of uniform particles of 10–30 nm (by scanning electron microscopy (SEM) studies) having high surface area. The ferrite synthesized from such nanoparticle size -Fe₂O₃ gave a porosity of 25% with grains ranging from 0–3 m. On the other hand, MgFe₂O₄ obtained from commercial alpha-Fe₂O₃ grains (of 1–2 m size) gave particles of 0–6 m with a porosity 42%. Saturation magnetization values 922–1168 G are found for MgFe₂O₄ from -Fe₂O₃ source while the alpha-Fe₂O₃ source gave the lowest value, 609. The Curie temperature, T_c, from magnetic susceptibility, initial permeability and resistivity measurements indicated a highest T_c of 737 K for MgFe₂O₄ from alpha-Fe₂O₃, while lower values are found for the ferrite prepared from -Fe₂O₃.     

The frictional behaviour of LiF single crystals

A study has been made of the influence of initial surface roughness, renewable and non-renewable surface contaminants, and irradiation hardening on the coefficient of friction for one LiF single crystal (A) sliding on another (B) in {100}_A<010>_A{100}_B 010_B orientation at 295 K. The normal load was 1 N, the nominal contact pressure 0.1 MPa, the sliding velocity 0.2 to 0.6 mm sec^–1, and the amplitude of the (reciprocate) motion a few millimetres. Any influence of non-renewable contaminants persisted only for cumulative relative displacements 0.1 m, and that of micrometre-scale initial surface roughness only for a few metres. At steady state in the presence of renewable contaminants the coefficient of friction varied only from a high of 0.45 in ultra-high vacuum ( 7.5 × 10^–8 Pa) and dry nitrogen-rich air ( 10⁵ Pa, relative humidity 15%) to a low of 0.38 in moist nitrogen-rich air ( 10⁵ Pa, relative humidity 50%). Irradiation hardening had no effect on the coefficient of friction at steady state. The worn surfaces created by steady-state sliding always exhibited a grooved topography partly obscured by more-or-less adherent layers of variously consolidated equiaxed debris particles 100 nm in size. Owing to the action of image forces, these particles contained no dislocations. It is suggested that the coefficient of friction was determined at steady state by the stress needed to shear these tiny particles past one another as near-rigid bodies.     

Dielectric properties of LiF single crystals

The dielectric constant,, loss, tan, and a.C. Conductivity, , of high quality single crystals of LiF have been measured in the frequency range 10² to 10⁵ Hz and in the temperature range 30 to 400° C. of LiF at 30° C is 8.3 and is frequency independent; tan was below detection level (tan<0.0001). increases slowly with temperature up to about 150° C (frequency independent region) beyond which it rises rapidly, being frequency dependent with larger values at lower frequencies. A similar behaviour is exhibited by tan . The activation energy for conduction in the high temperature region is calculated to be 0.93 eV from log against 1/T graphs. Log () against 1/T plots (where is the difference between the dielectric constant at any temperature and the value at room temperature, 30° C) yields an activation energy value of 1.0 eV in the high temperature region. This activation energy is found to be independent of frequency. This agreement between the two activation energy values, one for the a.c. conductivity and the other for the increase with temperature. This suggests that the same charge carriers are responsible for the two processes in the high temperature region.     

Particle size control of 1,3,5-triamino-2,4,6-trinitrobenzene by recrystallization from DMSO

Batches of up to 46g of the insensitive explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) have been recrystallized from DMSO in an effort to prepare larger particle-size material for recycling previously-used TATB and also for use in special formulations. The first part of the study investigated the conditions required to shift the particle-size distribution maximum from 50–70 m to several hundred micrometres in diameter. Distributions peaking at 200–246 m were successfully produced by varying the cooldown rate and degree of agitation during cooling. The second part of the study emphasized regeneration of the standard 50–70 m distribution from submicron size (ultrafine TATB) particles. The distributions peaking at 76–88 m, 27–31 m, and 15–17 m, successfully bracketed the target particle sizes, were grown by changing the degree of solution saturation. The choice of saturation temperature for the TATB/DMSO solution was based on earlier small-scale recrystallization and solubility work.     

Effect of Air Oxidation on the Thermophysical Properties of a Zirconium Alloy

The results of an experimental study of the effect of air oxidation on the temperature dependence of the heat capacity and normal spectral emissivity (for the wavelength of 0.65 m) of the zirconium alloy E635 (1.3 mass% Sn, 0.3 mass% Fe, 1 mass% Nb) are presented. The subsecond resistive pulse heating technique has been used. The samples were heated in ambient air by single and multiple cyclic pulses.     

New high permittivity and low loss ceramics in the BaO–TiO2–Nb2O5 composition

New dielectric ceramics with formula BaTi3Nb4O17 and Ba6Ti14Nb2O39 have been prepared and characterized. BaTi3Nb4O17 was densified to 92% of TD after firing at 1310 °C for 4 h. However, Ba6Ti14Nb2O39 fired under optimized conditions (1260 °C for 4 h) showed only 85% TD together with secondary phase. The crystal system of both of the compositions is orthorhombic. The BaTi3Nb4O17 has r56, Qu2100 (at 4.402 GHz), f+86 p.p.m. K-1 and Ba6Ti14Nb2O39 as r50, Qu650 (4.359 GHz) and f+165 p.p.m. K-1. © 1998 Kluwer Academic Publishers     

On crystal alignment in polycrystalline YBCO materials based on TEM observation

Previously, we have observed that parallel lattice fringes show up regularly in MTG samples, indicative of good alignment of unit cells in strips of size 100 nm × a few times 100 nm. We have prepared films ( 80 nm) for TEM observation using YBCO material fabricated by the conventional solid-state reaction method in order to study the crystal texture in a length scale from a few 100 nm down to 3 Å. Parallel lattice fringes are observed to stretch across a cross-section 0.3×0.5 m². Cross lattice fringes intersecting at 90° and 45° are observed as rare events. This result suggests that the degree of alignment in the crystal texture is higher than expected inside a grain of size a few m.     

Continuous,martensitic nature of the transition ß→γ Li3PO4

The transformation Li₃PO₄ shows characteristics of both continuous and martensitic transformations. Below 340° C, no detectable transformation occurs; between 340 and 410° C, the transformation goes only partially to completion; above 410° C the transformation rapidly goes to completion. At any temperature in the range 340 to 410° C, transformation proceeds rapidly in the initial stages to attain a certain degree of transformation. With prolonged isothermal heating or grinding of the samples, further transformation does not occur. The reverse transformation could not be effected under normal, dry conditions.     

Change of mechanical properties of rigid poly(vinylchloride) during photochemical ageing

Thermally stabilised, unpigmented, rigid PVC samples, were exposed to accelerated photoageing at 40°C, 55°C and 70°C. The concentration profiles of photoproducts were determined on microtome slices ( 20 m) parallel to the irradiated surface using IR (carbonyls) and UV (polyenic double bonds) spectrophotometry and by steric exclusion chromatography (Mn and Mw). They indicate that carbonyls and chain scissions predominate only in a thin superficial layer whereas polyconjugated double bonds and crosslinks predominate in a subcutaneous layer (300–400 m). Tensile measurements show that the ultimate elongation decreases after an induction period whose duration is a decreasing function of temperature. The change of mechanical behaviour can be described in terms of a ductile-brittle transition shift mechanism in which crosslinking plays an important role. A tentative explanation of a such crosslinking induced transition is proposed in the discussion.     

Influence of stabilizers on Na-β′′-Al2O3 phase formation in Li2O(MgO)-Na2O-Al2O3 ternary systems

The influences of stabilizers on - and -Al2O3 phase formations in Li2O(MgO)-Na2O-Al2O3 systems were investigated. When stabilized with 4MgCO3Mg(OH)25H2O, most of the -Al2O3 phase formed below 1200°C and further - to -Al2O3 transformation with an increase of temperature was not observed. On the other hand, when stabilized with Li2CO3,-Al2O3 formation occurred by two steps. First, -Al2O3 was partly formed below 1200°C, and, second, noticeable transformation from -Al2O3 to -Al2O3 occurred at higher temperature ranges. It was shown that transient eutectic liquid in the Li2O-Na2O-Al2O3 system promoted the - to -Al2O3 transformation at higher temperatures. Uniform distribution of both Mg2+ and Li+ stabilizing ions enhanced -Al2O3 formation at low temperatures. In the Li-stabilized systems, however, homogeneous distribution of Li+ ions hindered both the formation of transient eutectic liquid and the second - to -Al2O3 phase transformation at high temperatures.     

Experimental investigation of the heat capacity of the ternary alloy Nb-Ge-Si at low temperatures

The article describes several series of measurements of the heat capacity of a massive specimen of the ternary alloy Nb-Ge-Si from 4 to 35°K containing 8 at.% Si.Notation _D Debye temperature - C_p specific heat capacity at constant pressure - a, b, c coefficients - T temperature - T_c temperature of superconducting transition - magnetic susceptibility - increment of magnetic moment Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 42, No. 1, pp. 72–75, January, 1982.     

Low temperature internal friction on γ-irradiated polyvinylidene fluoride (PVDF)

A least-squares fitting of the below room temperature part of the internal friction spectra, obtained by the torsion pendulum technique on as-received and-irradiated (up to 1 Grad) strips and fibres of polyvinylidene fluoride [-(CH₂-CF₂-) _n -.; PVDF] by a superposition of single Debye functions, reveals that the spectral component features are determined not only by purely amorphous chain characteristics but also by the dosedependence of crystallinity. A careful analysis of the relaxation spectra confirms that at least one relaxation effect (236 K) is created upon irradiation. The analysis of the dose dependence of the characteristics of the (glass transition; 220 K) and _u (apparent upper glass transition; 270 K) relaxations, suggests the probable influence of crystallinity on the molecular motion in the amorphous phase. The increase of the intensity of the relaxation (190 K) is related to the irradiation-induced crystallite degradation.Fellow of the Interuniversitair Instituut voor Kernwetenschappen     

Reconfirmation of the existence of δ2 in dental amalgams

The 90°C endotherm peak in the thermogram of a conventional dental amalgam is due to two overlapping peritectic transitions involving SnHg-2 and HgAgSn-1, respectively. The aim of the present study is to separate these two events and thus confirm that the 2 phase is indeed a part of an amalgam microstructure. The materials used in this study were: sample 1, 1+1 wt% Sn; sample 2, 1+2 wt% Sn; sample 3, 1+3 wt% Sn; and sample 4, a commercial conventional amalgam with 48 wt% Hg. In powder form, they were exposed to 1 wt% NaCl solution at 37 °C for up to 45 days. At 15 day intervals, samples were withdrawn from the solution, washed, dried and then characterized by the differential scanning calorimetry (DSC) technique. Corrosion of Sn from ternary 1 matrix and intergranular 2 in respective materials during their exposure to the NaCl solution led to the following: (a) a progressive increase in 11 transition temperature in samples 1 and 2; (b) a gradual decrease in 2 peak and its disappearance in sample 2; and (c) in samples 3 and 4, initial splitting of the 90°C peak into two distinct endotherms associated with 2 and 1, respectively. Continued corrosion of the last two materials produced further changes in 2 and 1 in a manner similar to that seen in samples 1 and 2. On the basis of these observations, we have concluded that the 90°C endotherm is a valid indicator of the existence of the 2 phase in dental amalgams.     

CO/pH2: A Molecular Thermometer

We utilize reversible temperature dependent changes in the IR absorption spectrum of CO molecules isolated in solid parahydrogen (pH₂) to probe bulk temperature changes during rapid vapor deposition. The intensity of a well resolved feature near 2135 cm^–1 increases monotonically with temperature over the 2 to 5 K range. The thermally populated initial state of this transition lies 12 K above the CO/pH₂ ground state. During the deposition of 100 ppm CO/pH₂ samples, we detect temperature gradients 10 K/cm in 0.1 cm-thick samples subjected to heat loads 10 mW/cm². The resulting estimated thermal conductivity (TC) is 3(±2) mW/cm-K, averaged over the 2 to 5 K region. This value is 1000 times lower than the TC of single crystal solid pH₂, and 10 times lower than previously measured for pH₂ solids doped with 100 ppm concentrations of heavy impurities [Manzhelii, Gorodilov, and Krivchikov, Low Temp. Phys. 22, 131 (1996)]. We attribute this abnormally low TC to the known mixed fcc/hcp structure of the rapid vapor deposited solids.     

Nickel aluminide (Ni3Al) fabricated by reactive infiltration

Nickel aluminide Ni₃Al in the single phase form, with grain size 10 m, porosity 5%, tensile strength 425 MPa, modulus 92 GPa and ductility 9.5% at room temperature, was fabricated by reactive infiltration at 800 °C of liquid aluminium into a porous preform containing 78 vol % nickel and made by sintering 3–7 m size nickel particles. Without sintering, the preform contained 58 vol % nickel and reactive infiltration resulted in an aluminium-matrix NiAl₃ particle ( 50 m size) composite and extensive growth of Ni-Al needles from the preform to the excess liquid aluminium around the preform.     

Some observations on the deformation characteristics of bulk polycrystalline zirconium hydrides

The deformation behaviour of bulk polycrystalline zirconium hydrides in the composition range ZrH_1.27 to ZrH_1.66 has been investigated by compressive loading at temperatures between room temperature and 500° C. Single-phase -zirconium hydride is brittle below 100° C. Analyses of slip traces on specimens deformed at temperatures between 100 and 250° C have shown that the glide planes are {111} types. The deformation characteristics of and ( + ) alloys at temperatures between 100 and 500° C are consistent with the hydrogen vacancies in the -phase providing significant lattice friction to the movement of dislocations in the zirconium lattice of the hydride structure. The room temperature fracture stress of ( + ) alloys increases with the volume fraction of the -phase and this can be related to the resistance offered by platelets to the propagation of cleavage cracks in the matrix. In a ( + + ) alloy the resistance to crack propagation at room temperature is further increased by the soft -zirconium phase.     

Spherical particle preparation of Na+-ion conductive Na4Zr2Si3O12 by mist pyrolysis

Spherical polycrystalline particles of Na₄Zr₂Si₃O₁₂ (NZS) have been prepared by a mist pyrolysis technique with alkoxide ethanol solution. Firing the precursor particles, obtained by pyrolysing at 550 °C, yielded the NZS with the highest S/N ratio and the least second phase at 1000 °C. Each spherical particle with an average size of 0.5 m was composed of primary particles of 0.1 m diameter after firing. Comparing the particle formation process with the sol-gel, alkoxide pyrolysis and the aqueous solution processes, it would appear that the primary particle size of the mist-pyrolysed NZS depends on the size and residual alkyl group content of the precursor polymers.     

The thermal reactions of montmorillonite studied by high-resolution solid-state29Si and27Al NMR

Only very slight changes are observed in the²⁹Si and²⁷Al solid-sate magic-angle spinning NMR spectra of a montmorillonite containing almost equal numbers of octahedral aluminium and magnesium ions when its interlayer water is driven off by heating. The²⁹Si NMR spectra are unaffected by dehydroxylation which begins at 450° C, but the²⁷Al spectra show a decrease in total intensity, possibly due to the formation of 5-coordinated aluminium, with a slight increase in the intensity of the tetrahedral aluminium resonance. On the basis of these results, a structural model is proposed for the dehydroxylate phase and its formation mechanism is discussed. The destruction of the dehydroxylate X-ray pattern at 850° C and the subsequent recrystallization of the high-temperature products (-quartz, enstatite and high-cordierite at 1100° C;-cristobalite, enstatite and sapphirine at 1200° C) is accompanied by changes in the silicon and aluminium NMR spectra and in the⁵⁷Fe Mbssbauer spectra which are fully consistent with the known structural features of these phases.